1. FIELD OF THE INVENTION
The present invention relates to a process for polymerizing an olefin. More particularly, it relates to a process for polymerizing an olefin in the presence of a novel catalyst system comprising a titanium compound and a zirconium compound.
2. DESCRIPTION OF THE PRIOR ART
It has been proposed to prepare a polyolefin having a wide molecular weight distribution using a catalyst system comprising a titanium compound and a zirconium compound. For example, it has been proposed to use a catalyst system comprising triisobutylaluminum and a catalytic component obtained by reacting magnesium metal, an alcohol, titanium tetraalcoholate, and zirconium trichloride in Japanese Patent Publication No. 46387/1976, and a catalyst system comprising triisobutylaluminum and a catalytic component obtained by reacting magnesium diethoxide, titanium tetraalcoholate or titanium tetrachloride and zirconium tetraalkoxide or zirconium tetrachloride in Japanese Unexamined Patent Publication No. 138785/1976.
When an olefin is polymerized in the presence of such catalyst system, a polyolefin having a wide molecular weight distribution and an improved processability can be obtained, however, the characteristics are not satisfactory.
On the other hand, in order to prepare a polyolefin having a wide molecular weight distribution and an improved processability by a process of elimination of a catalyst residue separation step, it is required to use a catalyst system having high activity of the catalyst. That is, when the activity of the catalyst is high, the catalyst residue in the resulting polymer is small and a polyolefin having high quality is obtained. However, in order to attain high activity of the catalyst by using the conventional catalyst system, the processability of the resulting polyolefin is inferior and it requires a long polymerization time and high polymerization pressure to reduce productivity and it requires to use a polymerization reactor having high pressure durability.
The inventors have studied on these problems and have found that a polyolefin having a wide molecular weight distribution and an improved processability can be obtained by using a catalyst system comprising a specific titanium compound and a specific zirconium compound in high productivity and high activity of the catalyst under a condition of non-requirement durability of a polymerization reactor. The present invention has been attained by the finding.